Evidence for direct detection of ionizing particles in SiPMs 14th Topical Seminar on Innovative Particle and Radiation Detectors: IPRD16 Evidence for direct detection of ionizing particles in SiPMs R. Santoro1, M. Caccia1, S. Gentile2 and S. Veneziano3 1) Università degli Studi dell’Insubria, Dip. Scienza ed Alta Tecnologia - COMO 2) Università LA SAPIENZA, Dip di fisica - ROMA 3) INFN – ROMA

Introduction SiPM is a well know light sensitive detector Different teams are studying a protocol to qualify the detector according to the specific applications (i.e. V. Arosio’s Talk ) The typical measurements included in the protocols are: Dark Count Rate, Cross Talk, After pulse, Photon Detection Efficiency, Gain, Spectral response This talk will focus on the SiPM sensitivity to ionizing particles, an effect that would be better to consider for those applications were both light and ionizing particles are present R. Santoro IPRD16, 3 - 6 October 2016, Siena (Italy)

Experimental setup All the results shown in this talk are based on the following setup: Caen desktop control and amplification unit (SP5600A) 3-stage amplification with leading edge discrimination Caen desktop digitizer (DT5720) 250 Ms/s sampling rate, 2V dyn-Range and 12bit resolution Hamamatsu SiPMs (different area and different pitch size) The analysis is based on A series of measurements used to qualify the SiPMs in terms of DCR and Xtalk Spectra analysis based on data collected with and without 90Sr R. Santoro IPRD16, 3 - 6 October 2016, Siena (Italy)

SiPM qualification in the dark Dark Count and Xtalk measurement @ 15°C typical Signal Threshold scan SiPM (S12572-050) 3x3 mm2 with 50μm pixel size Vop=2.6V over breakdown DCR = 43 KHz Xtalk = 45 % (Ntrigger @ 1.5ph-e / Ntrigger @ 0.5ph-e ) R. Santoro IPRD16, 3 - 6 October 2016, Siena (Italy)

SiPM qualification in the dark Dark Count and Xtalk measurement @ 15°C typical Signal Threshold scan SiPM (S12572-050) 3x3 mm2 with 50μm pixel size Vop=2.6V over breakdown DCR = 43 KHz Xtalk = 45 % (Ntrigger @ 1.5ph-e / Ntrigger @ 0.5ph-e ) If I measure the mean value and its std in each step … Zoom R. Santoro IPRD16, 3 - 6 October 2016, Siena (Italy)

SiPM qualification in the dark Dark Count and Xtalk measurement @ 15°C typical Signal Threshold scan SiPM (S12572-050) 3x3 mm2 with 50μm pixel size Vop=2.6V over breakdown DCR = 43 KHz Xtalk = 45 % (Ntrigger @ 1.5ph-e / Ntrigger @ 0.5ph-e ) … I have a different way to show the threshold scan (staircase) R. Santoro IPRD16, 3 - 6 October 2016, Siena (Italy)

SiPM qualification in the dark: CrossTalk If you deal with low Xtalk probability, any approximation is good, while… … if you are looking for small effects and you have large Xtalk probability, a proper xTalk description is mandatory! Data-sheet Overvoltage in use corrected for ΔT = 10°C R. Santoro IPRD16, 3 - 6 October 2016, Siena (Italy)

SiPM qualification in the dark: CrossTalk If you deal with low Xtalk probability, any approximation is good, while… … if you are looking for small effects and you have large Xtalk probability, a proper xTalk description is mandatory! R. Santoro IPRD16, 3 - 6 October 2016, Siena (Italy)

SiPM qualification in the dark: CrossTalk If you deal with low Xtalk probability, any approximation is good, while… … if you are looking for small effects and you have large Xtalk probability, a proper xTalk description is mandatory! Geometrical Chain Branching Poissonian Definitely better! R. Santoro IPRD16, 3 - 6 October 2016, Siena (Italy)

SiPM response to a β source Threshold scan without source compared to threshold scan with 90Sr The 2 trends are scaled for the counting rate measured at 0.5 ph-e An excess of counting rate at higher threshold is quite evident R. Santoro IPRD16, 3 - 6 October 2016, Siena (Italy)

SiPM response to a β source Threshold scan without source compared to threshold scan with 90Sr The 2 trends are scaled for the counting rate measured at 0.5 ph-e An excess of counting rate at higher threshold is quite evident I’m using the Borel distribution to check if the 90Sr data set has an extract effect with respect to the DCR This is a clear indication that the response to a β source cannot be justified with an high Xtalk probability R. Santoro IPRD16, 3 - 6 October 2016, Siena (Italy)

Spectrum method Digitized Signal Typical Signal Typical Spectrum When a cell in the SiPM is fired, an avalanche starts with a multiplicative factor of about 105-106 The analogue signal is proportional to the number of fired cell Typical Signal Digitized Signal If integrated into a fixed gate, it is proportional to the number of fired cells Typical Spectrum A small number of fired cells allows to see the different peaks, but this effect disappears when the cells number increases The peak-to-peak distance allows to measure the light intensity even when the peaks are not distinguishable anymore R. Santoro IPRD16, 3 - 6 October 2016, Siena (Italy)

SiPM response to a β source Spectrum without source compared to spectrum with 90Sr Trigger set at 16mV (≈1.5 ph-e) The two spectra are normalized in time before the bin-to-bin subtraction R. Santoro IPRD16, 3 - 6 October 2016, Siena (Italy)

SiPM response to a β source Spectrum with 90Sr after background subtraction Fit performed using a fat tail distribution: a Moyal function* (Gaussian Landau convolution) Most Probable Value = 22.4 ph-e Assuming a cross-talk probability of 60%, we get a mean cluster size of ≈ 9 cells (Npixels=MPV*(1-xTalk)) *  J. E. Moyal “Theory of Ionization Fluctuations” Phil. Mag. 46 (1955), 263 R. Santoro IPRD16, 3 - 6 October 2016, Siena (Italy)

Is the effect dependent on SiPM pitch size? Threshold scan without source compared to threshold scan with 90Sr SiPM (S12572-010) 3x3 mm2 with 10μm pixel size The data are normalized wrt the trigger rate at the same threshold. The effect is still visible but we cannot be quantitative The cluster size cannot be measured because we don’t see the peak-to- peak distance R. Santoro IPRD16, 3 - 6 October 2016, Siena (Italy)

Same analysis with new sensors (just arrived!) Threshold scan without source VS threshold scan with 90Sr Detector size 1.3 x 1.3 mm2, (Hamamatsu S13360-1350PE / S13360-1325PE ) Two different pitch sizes (50 and 25 μm) Low xTalk probability New technology (we don’t know if they can be compared with the previous) XT=3% XT=1.4% Pitch size = 50 μm Pitch size = 25 μm R. Santoro IPRD16, 3 - 6 October 2016, Siena (Italy)

Same analysis with the new sensors (just arrived!) Spectrum without source VS spectrum with 90Sr Detector size 1.3 x 1.3 mm2 (Hamamatsu S13360-1325PE) No background subtraction is required XT = 1.4% Ncells≈ 6.74 Pitch size = 25 μm Background 90Sr Threshold Comparable with the previous sensor R. Santoro IPRD16, 3 - 6 October 2016, Siena (Italy)

Looking at the digitized signal Detector size 1.3 x 1.3 mm2, (Hamamatsu S13360-1325PE) xTalk probability 1.4 % Signal digitized at 250 Ms/s Typical DCR Event Typical Event with 90Sr R. Santoro

Looking at the digitized signal Detector size 1.3 x 1.3 mm2, (Hamamatsu S13360-1325PE) xTalk probability 1.4 % Signal digitized at 250 Ms/s Primary Avalanche Delayed Avalanches Typical Event with 90Sr Filter applied to count the peaks R. Santoro

Time distribution analysis (DCR) Detector size 1.3 x 1.3 mm2, (Hamamatsu S13360-1325PE) xTalk probability 1.4 % Signal digitized at 250 Ms/s Typical DCR Event Number of delayed avalanches distribution plot Primary Avalanche Peak Height distribution plot MPV ≈ 1phe MPV ≈ 0 R. Santoro

Time distribution analysis (90Sr) Detector size 1.3 x 1.3 mm2, (Hamamatsu S13360-1325PE) xTalk probability 1.4 % Signal digitized at 250 Ms/s Typical Event with 90Sr Primary Avalanche Peak Height distribution plot Number of delayed avalanches distribution plot MPV ≈ 4 phe MPV ≈ 2 R. Santoro

Conclusions and Outlook The SiPM sensitivity to direct detection of ionizing particles has been shown The effect has two main components Cluster effect: a certain number of cells start the avalanche at the same time Delayed effect: a certain number of avalanches start after the primary ones A more exhaustive analysis is on the way although a collaboration with SiPM producers will be beneficial to allow the proper description of the observed phenomena R. Santoro IPRD16, 3 - 6 October 2016, Siena (Italy)

Spares R. Santoro IPRD16, 3 - 6 October 2016, Siena (Italy)

Spectrum with / without source Spectrum without source compared to spectrum with 90Sr Trigger set at 16mV (≈1.5 ph-e) The two spectra are normalized in time before the bin-bin subtraction R. Santoro IPRD16, 3 - 6 October 2016, Siena (Italy)

Is the effect sensible to the SiPM pitch size? SiPM (S12572-010) 3x3 mm2 with 10μm pixel size Fit with a fat tail distribution: a Moyal function (Gaussian Landau convolution) Measured MPV = 250 a.u. the cluster size cannot be measured since don’t have the peak-peak distance χ2/dof = 2.95 Signal depends on the cell capacitance Assuming all is the same except the area, I presume the signal to be ≈25 time smaller If this assumption is correct I could use the previous dpp and I could make a guess on the MPV in ph-e If in addition I presume to have the same xTalk, the expected mean cluster would be MPV*(1-xT)= 11 cells Really too much even for an educated guess! R. Santoro IPRD16, 3 - 6 October 2016, Siena (Italy)

New sensors (just arrived!) Spectra with 90Sr Smaller detector size (1.3 x 1.3 mm2) Different pitch sizes (25 and 50 μm) Low xTalk probability New technology (we don’t know if they can be compared with the previous) Pitch size = 50 μm Pitch size = 25 μm XT = 3% Ncells≈ 6.8 XT = 1.4% Ncells≈ 6.74 R. Santoro IPRD16, 3 - 6 October 2016, Siena (Italy)